Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 497,200
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,509,800
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	395,300
Available-for-Sale Amortized Cost, After Ten Years	176,700
Amortized Cost	2,579,000	$ 2,305,200
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	496,000
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,504,900
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	395,500
Available-for-Sale Estimated Fair Value, After Ten Years	175,600
Available-for-Sale, Estimated Fair Value	2,572,000	2,270,700
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	44,900
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	44,200
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	24,300
Held-to-Maturity Amortized Cost, After Ten Years	3,600
Amortized Cost	117,000	406,800
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	44,800
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	44,100
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	25,500
Held-to-Maturity Estimated Fair Value, After Ten Years	3,600
Held-to-Maturity, Estimated Fair Value	118,000	$ 400,700
Callable Within One Year
Available-for-sale Securities and Held-to-maturity Securities
Amortized cost of investment securities callable after one year but within five years	121,300
Fair value of investment securities callable after one year but within five years	121,500
Investment Securities,Callable Structured, Amortized Cost Basis	2,000
Investment Securities,Callable Structured, Fair Value	$ 2,000